In a conventional power module used to control a large current, for example, in an electric vehicle, an electric train, a machine tool or the like, a metal-ceramic insulating substrate is fixed on one surface of a metal plate or a composite material called a base plate by soldering, and an electronic component such as a semiconductor chip is fixed on the metal-ceramic insulating substrate by soldering. Further, a heat radiation fin or a liquid-cooling jacket made of metal is attached to another surface (a rear surface) of the base plate via a heat conductive grease by screwing or the like. Patent Document 1 discusses that in the case of heat-bonding a ceramic substrate such as an aluminum nitride su+bstrate to a metal plate such as an Al plate or an Al alloy plate, a pressure of 8-100 kgf/cm2 is applied in a vertical direction on the ceramic substrate.
Since soldering of the base plate, the electronic component and so on to the metal-ceramic insulating substrate is performed by heating, warpage of the base plate is likely to occur due to a difference in thermal expansion coefficient between bonded members during the soldering. Further, the heat generated from the electronic component and so on is released to air, cooling water or the like by the heat radiation fin or the cooling jacket (radiator) via the metal-ceramic insulating substrate, the solder and the base plate, so that when warpage occurs in the base plate, the clearance when attaching the heat radiation fin or the cooling jacket to the base plate increases, thus extremely decreasing the heat radiation property.
Hence, for example, Patent Document 2 discloses a manufacturing method of a liquid-cooled integrated substrate and a liquid-cooled integrated substrate in which boding of a metal circuit board and a metal base plate to a ceramic substrate is performed by a molten metal bonding method and boding of the metal base plate and a radiator is performed by a braze-bonding method, and heat-bonding of them is performed in a state of pressurization at a predetermined surface pressure obtained by an expression decided according to a second moment of area of the radiator to reduce warpage (shape distortion) as the liquid-cooled integrated substrate and thereby has excellent strength and heat radiation performance.
Further, Patent Document 3 discloses that warpage with temperature change is prevented by directly bonding a metal layer having an average grain diameter of its crystal grain of 0.5 μm or less and a ceramic substrate with their surfaces in an active state.